Technical Field
For the production of a book or printed product, individual pages and/or sheets are printed and collated to form a loose book block. The loose pages and/or sheets within the stack formed in this way are aligned, relative to each other, in a perfect binder. The pages or sheets are then held in place with a clamp and moved across processing stations in which the spine of the printed product is machined and glue is applied.
Digital printing technology has made it possible to print and collate the pages and/or sheets in series, such that they respectively contain the content of a complete book or printed product. These loose stacks of pages and/or sheets are not very suitable to be transported or stacked without falling apart or without sliding around internally.
For that reason, an auxiliary gluing together of the individual pages and/or sheets has proven suitable to join, respectively integrate, the individual parts of a book block to form a single book block.
This auxiliary gluing together can take place in different ways. One proven technique is to apply drops of cold glue to the outside surfaces of the pages and/or the sheets in the printed product section. The individual pages and/or sheets, which are stacked one above the other, are glued together with these cold glue drops and thus form a relatively stable book block which is easy to handle during the further process for producing a bound book.
To apply the cold glue drops to the paper, nozzle valves are used which, however, are notorious for becoming dirty and thus frequently fail to meet the final objective.
Prior Art
European patent document EP 0719 592 A2 discloses a nozzle unit for processing flowable substances, in particular glue. To prevent glue residues from hardening inside the nozzle unit during a temporary interruption in the operation, a liquid, and in particular water, is squirted into the nozzle channel so that glue is flushed from an end section on the nozzle outlet. By closing off the nozzle channel, a column of water remains inside the nozzle channel, thereby sealing it against the outside. For a renewed use of the nozzle, the closing device is removed and the water column is pushed out by the renewed pumping of glue. A device of this type solves the problem of plugging up the nozzle outlet opening with hardened glue during breaks in the operation. However, the device cannot prevent the nozzle from becoming dirty in the area of the outlet opening. Glue residues collect at the outlet opening, harden thereon, and then deflect the flowing-out glue stream which further promotes contamination of the nozzle, thus causing the nozzle to be plugged within a short time. A secure glue exit can be ensured only if the glue residues are periodically removed. The length of time between the cleaning intervals can be very short, depending on the type of glue that is used and the prevailing environmental conditions.
An apparatus for applying flowable substances, in particular glue, to a substrate is disclosed in the German patent document DE 199 36 670 C1. This apparatus has a fixed support arm and a thereto connected metering valve. According to this document, the metering valve is arranged on the support arm, so as to pivot around an axis. During the pivoting movement from the working position to the resting position, the outlet opening on the nozzle of the metering valve slides over the cleaning segment of a rubber element and is thus freed of any adhering glue residues. In the resting position, the outlet opening is closed off by the smooth segment of the rubber element. The rubber element is preferably arranged on a spring-elastic holder. This arrangement prevents the clogging of the nozzle outlet opening during interruptions in the operation because the opening is closed off and the residual glue consequently does not dry out and harden inside the bore for the outlet opening. By moving the nozzle outlet opening across the cleaning segment of the rubber element, hardened or dried glue residues are scraped off the nozzle. However, this apparatus has the disadvantage that in order to wipe off the glue residues, the nozzle must be pivoted from the operating position to the resting position and back again. During this movement, the nozzle cannot be used for applying glue to the substrate. It has furthermore turned out that the glue residues relatively quickly dirty the rubber element and that this element must be cleaned frequently.
A similar apparatus is described in European patent document EP 2 006 030 A2. In that case, the metering valve with the outlet opening for the glue, paint or varnish is arranged immovably. A pivoting outside closing device can be moved between a resting position and an operating position, wherein the outlet opening on the metering valve is closed off during the resting position by the outside closing device. This apparatus also has the same disadvantages as the apparatus disclosed in the DE 1 99 36 670 C1.
An apparatus and a method for dispensing a moisture-hardening glue is known from European patent document EP 2 248 598 A1. The apparatus is provided with several nozzle openings which can be operated in two operating modes. In one operating mode, not all nozzle openings are operational, so that precautions must be taken to ensure that the unused nozzle openings will not be gummed up. Residual glue remaining on the outlet opening of the nozzle comes in contact with the ambient air that contains moisture and thus hardens. The hardening of the glue is prevented with the aid of a flow chamber in the region of the nozzle opening by blowing a gas into this flow chamber, which then washes around the nozzle outlet opening containing the residual glue, thereby preventing the moisture in the air from coming in contact with the glue. This apparatus is designed for and has proven itself for use with moisture-hardening glues such as PUR (polyurethane reactive) glue and the like. However, this apparatus is not suitable for use with cold glues which harden through drying. Experiments have shown that using dry air or a gas to wash around the outlet opening of a nozzle valve which dispenses cold glue does not prevent the contamination of the nozzle with glue residue, but rather promotes it.